At present, spectrographs containing basic optical elements are mainly used to detect the incident electromagnetic radiation differentiated according to wavelengths, such basic optical elements being: an aperture for the entering polychromatic, usually divergent radiation beam, e.g. a slit. Furthermore, a collimating element which converts the divergent optical beam coming from the entrance aperture into a collimated beam—which is parallel. Further, a dispersive element, generally an element causing refractive dispersion (light refraction) or diffractive dispersion (light diffraction) of a polychromatic beam into monochromatic beams according to wavelengths. Further, it comprises a focusing element, which generates an image of the entrance aperture—usually a slit, at the site of a certain focal plane of the exit aperture, usually on the flat multi-channel radiation detector, in order to record simultaneously the radiation of a large number of wavelengths. The dispersive element in multi-channel detectors is usually a diffraction grating or a dispersive prism.
Notes on Raman spectroscopy: Raman spectroscopy is used to study the structure of Raman scattering molecules. Raman scattering is an inelastic scattering of the optical radiation on the molecules of the test substance, at which the frequency of the scattered radiation is shifted towards the frequency of the incident radiation by a value that corresponds to the rotational or vibrational transition in the molecule. A molecule can exist in states with energy that acquires only certain allowed values or energy levels. Vibrational or rotational transition is a change in the molecular state characterized by an abrupt change in the vibrational and rotational energy levels of the molecule. Vibrational energy level is a possible value of energy that is acquired by a molecule in the vibrational motion of its atoms around their equilibrium position. Rotational energy level is a value of energy that can be acquired by a molecule during its rotational movement around the axis passing through its center of gravity. These energy levels are dependent on the particular atomic structure and their spatial arrangement, i.e. on the type of the test substance.
Raman spectroscopy is used to measure inelastically scattered light (Raman scattering) on the molecules of the test substance. Scattered radiation has a different wavelength than the incident radiation, due to interactions of the radiation with vibrational and in some cases rotational states of molecules. The scattered radiation carries a large amount of information about the nature and structure of the test substance. Resonance enhancement of Raman scattering of biologically relevant substances (proteins, nucleic acids, and others) can be achieved in the ultraviolet region of the spectrum, at about 205 to 270 nm, to obtain additional unique information on these substances.
Raman scattering is a relatively weak phenomenon. The spectrograph for Raman spectroscopy is subject to high demands in terms of lens speed, i.e. the amount of radiation transferred to the detector. Further efforts are needed to achieve high resolution (0.03 to 0.06 nm) and spectral range of tens of nanometers.
The currently used and commercially available spectrographs (supplied by Horiba Jobin Yvon, Princeton Instruments, Andor) useful for Raman scattering in the UV region, are generally composed of two mirrors and a reflective diffraction grating. Alternatively, the optical assembly is complemented by an additional correction mirror, as described for example in the U.S. Patent Publication 2013/0182250 A1. The patent covers the design of mirror imaging spectrographs, in which the assembly of the collimating mirror, diffraction grating and focusing mirror is supplemented by at least one corrective aspheric mirror, which is intended to correct extra-axial imaging defects (aberrations), in particular astigmatism and coma. The advantage of the mirror systems is their achromaticity (absence of color defects) and a relatively high reflectivity. The disadvantages are the limited possibility of correction of other optical defects, in particular astigmatism and coma. Moreover, these spectrographs achieve maximum lens speed of only about f/4.
Better correction of optical defects, and thereby a higher lens speed, can be achieved using lens objectives. In the visible spectral range, these systems with the lens speed of up to f/1.8 and using transmission gratings (supplied by Kaiser, model Holospec f/1.8) are very well available. However, the situation in the ultraviolet spectral region is completely different, mainly due to the limited number of transparent optical materials. There is high demand on the design of lenses, thus increasing the complexity of the system and its cost. The scientific literature describes only one spectrograph operating in the UV spectral region and containing lens objectives as collimating and focusing elements, which achieves the nominal lens speed of f/2. This system, however, achieves spectral resolution of only 12-14 cm−1 and its throughput is limited by surface losses and vignetting (blocking of the outer parts of the beam) due to a large distance between the collimating and focusing objectives.